This invention relates to the field of alfalfa plants, and more specifically to an improved synthetic alfalfa variety and a method for producing such a synthetic variety.
Alfalfa (Medicago sativa) is an important and valuable forage and feed crop throughout the world. Alfalfa exhibits traits setting it apart from many other crop plants. It is an auto-tetraploid and is frequently self-incompatible in breeding. When selfed, little or no seed is produced, or the seed may not germinate, or when it does, it may later stop growing. Typically, fewer than five percent of selfed crosses produce seed. When a very small population is crossbred, inbreeding depression occurs, and traits of interest, such as quality, yield, and resistance to a large number of pests (e.g., seven or eight different pests), are lost. Thus, producing a true breeding parent for hybrids is not possible, which complicates breeding substantially.
Some sources indicate that there are nine major germplasm sources of alfalfa: M. falcata, Ladak, M. varia, Turkistan, Flemish, Chilean, Peruvian, Indian, and African. Tissue culture of explant source tissue, such as mature cotyledons and hypocotyls, demonstrates the regeneration frequency of genotypes in most cultivars is only about 10 percent. Seitz-Kris, M. H. and E. T. Bingham, In vitro Cellular and Developmental Biology 24 (10):1047-1052 (1988). Efforts have been underway to improve regeneration of alfalfa plants from callus tissue. E. T. Bingham, et. al., Crop Science 15:719-721 (1975).
Some methods for regeneration of alfalfa plants from tissue culture are described in U.S. Pat. No. 5,324,646 issued Jun. 28, 1994, which is hereby incorporated by reference. Certain other methods for generating alfalfa plants and seed are described in patent application Ser. No. 08/996,965 entitled xe2x80x9cIMPROVED ALFALFA LINE CALLED WL-C290 AND METHOD FOR PRODUCING SAMExe2x80x9d filed Dec. 23, 1997, and commonly assigned to the assignee of the present invention, which is hereby incorporated by reference.
Additionally, researchers believe that somatic embryogenesis in alfalfa is heritable, and is controlled by relatively few genes. Efforts at improving regeneration have thus been directed towards isolation of the genetic control of embryogenesis, and breeding programs which would incorporate such information. See, e.g., M. M. Hernandez-Fernandez, and B. R. Christie, Genome 32:318-321 (1989); I. M. Ray and E. T. Bingham, Crop Science 29:1545-1548 (1989).
Several genera of soilbome fungi can attack alfalfa seedlings, including Phytophthora, Pythium, Aphanomyces, Fusarium, and Rhizoctonia. Traditionally, Phytophthora and Pythium have been considered the most problematic pathogens, but recently Aphanomyces euteiches has been more widely recognized as a problem. According to one survey (referred to in Iowa State University""s Integrated Crop Management Issue IC-478(5)xe2x80x94Apr. 21, 1997 (see http://www.ipm.iastate.edu/ipm/icm/1997/4-21-1997/alfalfa.html)), Aphanomyces is more common than Phytophthora in Iowa soils, and these two fungi should be considered equal threats to seedlings. Seedling diseases should be suspected when emergence is poor and/or there are obviously stunted, discolored, or dead seedlings. Like other crops, alfalfa seedling diseases are more severe in wet conditions. Unlike other crops, alfalfa seedlings are relatively cold-tolerant, therefore, it is not necessary to delay planting to avoid disease unless conditions are too wet. The best way to avoid seedling diseases is to plant varieties with a resistant (R) or highly resistant (HR) rating to both Phytophthora and Aphanomyces.
Alfalfa planted in poorly drained fields can succumb to root-rot fungi diseases such as Pythium, Phytophthora and Aphanomyces. Pythium (damping off) damage can be reduced with an Apron-brand (Metalaxyl) seed treatment. Phytophthora root rot can affect alfalfa at any age (seedling to adult plant). However, many established alfalfa varieties have high resistance to Phytophthora. Recently, Aphanomyces has been associated with seedling blight and poor seedling establishment. Even alfalfa stands with high levels of resistance to Phytophthora root rot can perform poorly in wet soils because of Aphanomyces. Unlike Phytophthora, Aphanomyces mainly affects the plant during the seedling stage. Aphanomyces root rot can be confused with mild herbicide damage, because the plants remain chlorotic and stunted. An alfalfa seedling infected with Aphanomyces root rot usually does not collapse and decay, as in damping off or Phytophthora root rot.
According to Dr. Craig Grau, Extension Plant Pathologist, the alfalfa Aphanomyces population includes two virulence types; Race 1 and Race 2. Race 1 forms are said to not cause severe disease on commercial varieties with an R or HR rating to Race 1 Aphanomyces root rot. Race 2 isolates cause severe disease on Race-1-resistant varieties in lab tests. Race-2-resistant breeding lines should out-yield Race-1-resistant varieties if Race 2 Aphanomyces is present or predominant in the soil.
High-quality alfalfa is alfalfa that is high in crude protein, high in digestibility, and low in acid and neutral-detergent fibre. Yield in alfalfa is measured as compared to check varieties, where greater than about 104%-105% of the check variety is considered high-yield.
Thus, there is a need for a high-yield, high-quality alfalfa that is resistant to Race 2 Aphanomyces root rot.
The present invention provides a semidormant, Dormancy-Group-3-type synthetic alfalfa variety, wherein one embodiment is named WL-W316, that provides greater feed value and improved yields under a variety of environmental conditions and, in particular, better resistance to the wet soil pathogen Race 2 Aphanomyces root rot. The primary uses of this WL-W316 variety are hay, haylage, greenchop and dehydrated feed for livestock.
The present invention also provides a method for breeding and selecting alfalfa in order to obtain resistance or high resistance to certain alfalfa pests. In particular, this method obtains a semidormant alfalfa variety that is high in forage quality and resistant to Race 2 Aphanomyces root rot.
In one embodiment, the present invention includes seed of synthetic alfalfa variety designated WL-W316 and having American Type Culture Collection (ATCC) 10801 University Boulevard, Manassas, Va. 20110-2209, received Jul. 15, 1998, Accession No. 203064. In another embodiment, the present invention includes an alfalfa plant or its parts produced by the seed of synthetic alfalfa variety designated WL-W316 or produced by regenerable plant parts of such seed. In yet other embodiments, the present invention includes pollen or an ovule of the plant produced by the seed of synthetic alfalfa variety designated WL-W316.